Currently, all ordinary earphones are provided with a button used to switch between songs in music listening or answer a call for a conversation. Generally, most buttons are implemented in circuits by short-circuiting microphone (MIC) lines of earphones, that is, by connecting a button and a MIC in parallel. When the button is pressed, no voltage exists at two ends of the MIC because the MIC is short-circuited; when the button is not pressed, voltages exist in circuits of the two ends of the MIC. Therefore, when an earphone is inserted into a terminal device, such as a mobile phone or a computer, the terminal device may determine, by monitoring voltages at two ends of a MIC, whether a button is pressed, and thereby implement a corresponding function.
With continuous development of technologies, ordinary earphones gradually evolve into noise reduction earphones with a four-conductor plug. In a noise reduction earphone with a four-conductor plug, a MIC line needs to be used also as a power line (also referred to as a charge line). In this case, if the button is still implemented according to a circuit in an ordinary earphone, because one end of a parallel circuit is connected to a power supply, and the other end is grounded, when the button is pressed, the power line is directly short-circuited to ground, resulting in circuit burnout and causing a great potential risk.
Therefore, a problem to be solved urgently in the industry is how to detect a button on a noise reduction earphone with a four-conductor plug and further implement a corresponding function according to a detected state.